Electrical crystal unit



June 4, 1958 s. L. LEVY ELECTRICAL CRYSTAL UNIT Filed Jan. 6, 1956 Fig.5

4|) Fig 4 IN V EN TOR. STEPHEN L. LEVY Fig. 3

United.

2,840,710 ELECTRICAL CRYSTAL UNIT Stephen L. Levy, Bedford, Mass,assignor to Sylvania Electric Products inc, a corporation of Massachwsetts Application January 6, 1956, Serial No. 557,665

6 Claims. (Cl. 250-31) i the like at microwave frequencies.

For the mixer of superheterodyne receivers used at microwavefrequencies, it is common practice to employ a crystal diode, ratherthan a vacuum tube type mixer. Crystals, commonly although notnecessarily silicon, afford more favorable signal to noise ratios thanthe triode or pentode mixers used at lower frequencies. Furthermore,crystals by reason of their small physical size and simple configurationare better suited to effective incorporation within waveguide or coaxialcable transmission systems.

In such systems wherein a crystal mixer is employed, it is necessary toprovide suitable connections for the incoming signal (hereinafter termedthe-R. F. input), the oscillator signal (L. O. or local oscillator), andfor the output signal (I. F. signal) resulting from the beating ormixing of the oscillator frequency with the signal frequency. In theconventional coaxial crystal cartridge, the crystal is secured to oneface of a metal plug, termed the back plug, that is in direct metalliccontact with the shell. The cat whisker that contacts the crystal iscarried by a pin that extends through and is supported by an insulatingbead fitting within the shell.

In such conventional cartridges, it is necessary to employ a mountingfor the cartridge which enables the pin to serve as the input connectionfor the R. F. signal to the crystal, and also for the I. F. or videooutput from the crystal. This necessitates the use of an R. F. choke onthe output arm of the mounting, to keep the R. F. signalout ofthe inputto the I. F. or video amplifier. Since the choke is effective only overa relatively restricted frequency range, the use of an R. F. chokelimits the frequency range over which the assembly is operative, so thatbroad band operation is not satisfactory.

In addition to the problems involved in the R. F. choke, theconventional crystal cartridge necessitates special provisions in themounting for the separate I. F. output from the pin carrying thewhisker. The conventional crystal mounting also presents connectionproblems when the crystal is to be used as a simple detector.

Many of the limitations and problems encountered in using theconventional type of mixer crystal cartridge in superheterodynemicrowave receivers are avoided by using crystal cartridges of the typedisclosed by Theodore S. Saad in application Serial No. 484,763, filed-January 28, 1955, and assigned to the assignee of this invention. In theSaad application a crystal cartridge is disclosed which not onlysimplifies the mounting of the crystal in coaxial and waveguide systemsbut which also permits use of a wider range of operating frequencies.This is accomplished 'by insulating the metal plug holding the crystalfrom the metal shell of the cartridge so as to obtain a capacitancewhich will serve as a low impedance pass from the plug to the shell forthe R. F. signal and allow rates Patent the I. F. output to be taken atthe plug, thus eliminating the necessity of the R. F. choke at the I. F.output, and providing a crystal mixer capable of operating over abroader band of frequencies. A so-called D. C. return path forcompleting the I. F. circuit back to the center conductor or pin of theSaad cartridge may be afforded for example by extending the input probeconnected to the center conductor to the walls of the waveguide.However, this has the disadvantage of either drastically reducing theoperating bandwidth of the mixer or requiring manual tuning at thecrystal cartridge input.

In the above mentioned Saad application a cartridge construction isdisclosed which permits proper matching overa wide band of frequencieswithout tuning. This is accomplished by the use of a D. C. returnlocated within the chamber defined by the shell, plug, and insulatingbead, and generally is in the form of a small diameter wire extendingdirectly between, and electrically connecting, the center conductor andthe shell of the crystal cartridge. This arrangement has aifordedexcellent results for use at most frequencies at which the crystalcartridge itself is capable of satisfactory operation. However, it hasbeen found that at lower incoming signal frequencies there is sufficientleakage of the signal via the D. C. return to cause appreciableattenuation of the signal reaching the crystal and taking part in themixer action.

It is therefore an object of this invention to provide a coaxial crystalcartridge for use at microwave frequencies which makes possiblesimplified connection in a microwave system and which permits operationover a wide band of frequencies without objectionable variation withfrequency of input signal strength at the crystal.

It is a feature of this invention to provide a coaxial crystal cartridgein which the R. F. input is isolated from the output, the cartridgeincorporating a D. C. return path within the cartridge permittingoperation in the lower portion of the range of frequencies for which thecrystal is designed without excessive attenuation of the input signal.

A further feature of this invention is to provide a coaxial crystalcartridge in which an internal D. C. return path is provided to connectthe center conductor and the shell of the cartridge, the path being ofhigh impedance to reduce the loss of input signal at lower microwavefrequencies.

The objects of this invention are achieved through certain adaptationsand improvements of crystal cartridges of the type described in the Saadapplication. For example, although there is provided in the cartridgesof the present invention a D. C. return path connecting the centerconductor and the shell to complete the I. F. circuit associated withthe crystal output, the return, instead of being a direct short-pathconnection, is in the form of a conductor element having appreciableinductive reactance even at lower microwave frequencies, which offers ahigh impedance to the input signals, thereby reducing the loss of signalstrength through theD. C. return.. Generally the conductor elementconsists of a thin wire connected at one point to the center conductorof the cartridge and extending along a tortuous path, in cluding atleast one inductive turn, to a point of connection on the shell of thecartridge.

3: Q9 of Fig. 4 of a modification of the crystal cartridge of thisinvention mounted for use as a waveguide mixer.

Fig. 4 is a plan view of the arrangement shown in Fig. 3 in sectionalong line 4-4 of Fig. 3.

Fig. 5 is a sectional view of a further modification of the crystalcartridge of the type contemplated by the present invention.

The embodiment of my invention shown in Figs. 1 and 2 is a coaxialcrystal cartridge comprising a shell or outer conductor in the form of ahollow cylinder. Fitting snugly within the shell is a cylindricalinsulating head or support 11 through which extends a center conductorpin 12. The pin has a reduced tip portion 13 at one end to facilitatetelescoping connection with associated coaxial structure. The shell andthe pin are of appropriate dimensions to permit connection for exampleto standard coaxial cable and holders. A resilient cat whisker 14 with apoint for making rectifying contact with the crystal 16 is secured as bywelding or soldering at the opposite end of pin 12 from the reducedportion 13. The crystal or semiconductor element 16. usually silicon atmicrowave frequencies, is supported by a cylindrical metallic back plugto which the crystal is secured as for example, by soldering. The backplug is insulated from the cartridge shell by means of a layer or sleeveof insulating or dielectric material 20. Primarily to facilitateinsertion of the back plug assembly including the dielectric layer intothe shell a metallic ring or sleeve 17 is provided between thedielectric layer and the shell as shown. The plug and the sleeves and 17are suitably dimensioned so that the entire assembly fits snugly withinthe shell. The plug is provided with a suitable means for making anoutput connection. As shown in Fig. l, recess 18 is provided to receivethe center conductor of a coaxial connector. shell together with thedielectric layer form a condenser of appropriate capacitance to by-passthe R. F. signal from the crystal to the shell, but to act as arelatively high impedance to the I. F. signal from the crystal which ismade available at connection means 18 in the plug.

A wire 19, which is an extension of the cat whisker l4 beyond the pointat which the whisker is welded to the center conductor 12, extends in atortuous path from its point of connection with the center conductor to,a point of connection with the shell of the cartridge to serve as aninternal D. C. return path for any microwave circuit in which thecartridge may be employed. Although the conductor element constitutingthe D. C. return may be of any configuration having proper inductance, Ihave found that from the standpoint of fabrication and assembly the D.C. return conductor element 19 shown in Figs. 1 and 2 as a flat spiral,offers distinct advantages. The end of wire 19 is bent at its point ofcontact with the shell and the end portion is held in position betweenthe insulating bead and the shell. This connection is achieved at thetime of assembly by arranging the D. C. return element properly on theinsulating bead 11 and forcing the head into the position in the shellas shown in Fig. 1. 7

Figs. 3 and 4 illustrate a modified form of the crysta cartridge of thisinvention mounted to operate as a mixer in a system employingrectangular waveguide. The crystal cartridge 30 is essentially similarto the device shown in Fig. l in that it includes a shell 31 enclosing aconductive back plug 34 and insulating bead 32 supporting, respectively,silicon crystal 36 and center conductor or pin 33 to which pointed catwhisker 37 is welded as shown. As in the device of Fig. 1 the plug andbead are spaced apart within the shell to afford suitable rectifyingcontact between the whisker point and the crystal. A recess 38 isprovided in the back plug to afford connection for the I. F. outputsignal from the crystal. Unlike the cartridge shown in Fig. 1, the plugis separated from the shell only by the dielectric sleeve 35. Inaddition, the D. C. return element 39 includes only a single turn aboutthe center conductor be- The plug and tween its points of connectionwith the center conductor and shell. Still further, the return element,rather than being an extension of the cat whisker, is a separate lengthof wire welded or soldered at its terminal ends 39a and 39b to thecentral conductor and shell of the cartridge as best shown in Fig. 4.

The crystal cartridge 30 is mounted in a holder 40 which is attached asby Welding to a section of waveguide 41. Flange 42 is provided forconnecting the section of waveguide to the R. F. input and the localoscillator signal. A probe 43 is connected to the end of pin 33 andextends about half-way into the section of waveguide 41.

In operation the R. F. input signal and the local oscillator signal arefed into the rectangular waveguide at the flange 4-2. These inputsignals then pass via probe 43 into the crystal where the mixing orbeating action takes place. As a result of this mixer action energy ispresented at the back plug at the frequencies of the R. F. and localoscillator signals as well as the frequencies represented by the sum anddifference of these signals. Usually the I. F. output signal of thecartridge taken at connection means 38 is at the frequency of thedifference between the input signals. For example, assuming thecartridge 30 to be designed for operation with R. F. input frequenciesin the range of 1,000 to 12,000 rnegacycles per second (me. p. 5.), thedifference between the R. F. and local oscillator frequencies typicallyis of the order of 30 me. p. s. By virtue of the capacitative effectafforded by the assembly of the back plug, the shell, and the dielectriclayer 35, the energy of input signal and higher frequencies passesdirectly from the back plug to the shell. However, the value of thiscapacitance is such that the assembly offers a high impedance to the I.F. signal which is taken off a suitable connection at recess 38.Typically, for the range of frequencies referred to above, thecapacitance of the plug, shell and dielectric layer assembly is of theorder 712 f.

Normally the I. F. output energy as it is employed in further stages(not shown) is connected through an impedance network to ground. It islikewise essential that a ground or return path be provided for the D.C. and low frequency products of the mixer action. In devices of thetype disclosed in the above-discussed Saad application this has beenaccomplished by means of a short wire within the crystal cartridgeinterconnecting the center conductor and the shell of the cartridge.However, it has been found in employing cartridges of this type insystems involving input signals of lower microwave frequencies, e. g.1,000 to 3,000 me. p. s., that the input signals to the crystal areappreciably attenuated by loss through the return wire. Such attenuationis avoided in the arrangement shown in Fig. 3 by virtue of the novelconfiguration of the D. C. return element of wire 39 which is woundabout the center conductor 33 in noncontacting spiral fashion to form atleast one (as shown) or more complete turns or loops between itsterminal connections at the center conductor and shell. Apparently theeffect of this configuration of the wire is to produce suflicientinductive reactance in the D. C. return conductor so that appreciableimpedance is offered to the flow of energy at signal frequencies as lowas about 1,000 me. p. s. In any event it has been observed thatsubstantially improved signal to noise ratios are realized in cartridgesemploying my novel D. C. return as compared to cartridges in which ashort path direct return is used.

It will be obvious from the above explanation that the preciseconfiguration of the return conductor may be varied depending on thefrequency of input signals involved. Thus, if a spiral arrangement isemployed, it may be stated generally that an increase in the number ofturns will effect a decrease in signal loss at lower frequencies. Withinthe requirements of this invention, the cross-sectional shape or area ofthe return conductor has not been found to be critical. However, wire offrom 1.5 to 2.5 mils diameter has been found to be generally compatiblewith the small space normally available in crystal cartridges of thetype contemplated by this invention. Preferably the return element is inthe form of a flat spiral arranged within the shell with the centralconductor generally at the origin or center of the spiral.

In the crystal cartridges shown in Figs. 1, 2, 3, and 4 the cat whiskeris supported on the center conductor to which the input signal isapplied. It may be desired to interchange the positions of the catwhisker and crystal as illustrated by the crystal cartridge shown inFig. 5. In this case the crystal as is soldered to the inner end ofcenter conductor 54 while the cat whisker 57 is carried by the back plug51 which again is insulated from shell 50 by dielectric layer or sleeve52. The return conductor element 53 is shown as a fiat spiral similar tothe D. C. return element of the device of Pig. 1. However, in thisarrangement the return wire obviously cannot be an extension of the catwhisker. As shown in Fig. 5 it is a separate length of wire welded atone end to the center conductor and secured in contact with the shell atits other end by the tight fit of the insulating bead 55 against theshell. The return element in this case functions in the same manner asthe corresponding elements in Figs. 1, 2, and 3 in affording a path forreturn of the D. C. component of the input signals to the shell of thecartridge.

It should also be understood that Fig. 3 shows only one of the possibleapplications of this invention. The crystal cartridge has other useswhich will be obvious from the foregoing explanation of the invention,for example, as a mixer in a coaxial line or as a video detector. Someof these applications are shown in the aforementioned Saad application.

What I claim is:

1. A crystal cartridge comprising an outer conductor, cooperatingsemiconductor and rectifying connection elements, means supporting saidcooperating elements in fixed position with respect to said outerconductor, and means providing a direct current path from one of saidelements to said outer conductor including a conductor element having atleast one inductive turn in its length.

2. A crystal cartridge for microwave apparatus comprising asemiconductor element, an outer conductor, means for supporting thesemiconductor element in fixed position with respect to the outerconductor, an inner conductor having means making rectifying connectionwith said semiconductor element, and a direct current return elementextending in a tortuous path between, and electrically connecting, saidinner and outer conductors.

3. A crystal cartridge for microwave apparatus comprising a conductiveouter shell, cooperating crystal and cat-Whisker elements within theshell, a first conductive support on which one of said elements ismounted, a second conductive support on which the other of said elementsis mounted, means for holding said supports in said shell in position toobtain rectifying contact between the supported elements, and a directcurrent return element comprising a conductor having an inductive turnelectrically connecting the shell with one of said supports.

4. A coaxial crystal cartridge comprising a conductive shell, aconductive plug within the shell, a layer of dielectric material betweenthe plug and the shell, a crystal mounted on said plug, an insulatingsupport also within said shell and spaced from said plug, a pinextending through and fixed in position by said insulating support, acatwhisker secured to said pin and in rectifying contact with thecrystal, and a direct current return element extending in a tortuouspath between, and electrically connecting, the shell and thecatwhislser.

5. A coaxial crystal cartridge comprising a conductive cylindricalshell, a conductive back plug within the shell, a sleeve of dielectricmaterial between the shell and plug, an insulating bead also within theshell and spaced axially from said plug, a central conductor securedaxially in said bead with end portions on opposite sides thereof, acrystal mounted on said plug, a catwhisker secured to an end portion ofsaid central conductor and in rectifying contact with the crystal, and adirect current return element electrically connecting said centralconductor to the shell and extending in a spiral path from its point ofconnection with the central conductor to its point of connection withthe shell.

6. A coaxial crystal cartridge as in claim 5 in which said returnelement is located between the plug and bead and is in the form of afiat spiral conductor arranged with the central conductor generally atthe origin of the spiral.

References Cited in the file of this patent UNITED STATES PATENTS2,563,613 Ohl Aug. 7, 1951 2,642,494 Zaslavsky June 16, 1953 2.677.757Matare May 4. 1954

